The invention relates generally to improvements in thermometers and, more particularly, to electronic predictive thermometers for more rapidly obtaining accurate temperature measurements from a plurality of patient measurement sites.
It is common practice in the medical arts, as in hospitals and in doctors' offices, to determine the body temperature of a patient by means of a temperature sensitive device that measures the temperature and displays that measured temperature. One such device is a glass bulb thermometer incorporating a heat responsive mercury column that expands and contracts adjacent a calibrated temperature scale. Typically, the glass thermometer is inserted into the patient, allowed to remain for a sufficient time interval to enable the temperature of the thermometer to stabilize at the body temperature of the patient, and subsequently removed for reading by medical personnel. This time interval is usually on the order of two to eight minutes.
The conventional temperature measurement procedure using a glass bulb thermometer or the like is prone to a number of significant deficiencies. Temperature measurement is rather slow and, for patients who cannot be relied upon (by virtue of age or infirmity) to properly retain the thermometer for the necessary period of insertion in the body, may necessitate the physical presence of medical personnel during the relatively long measurement cycle, thus diverting their attention from other duties. Furthermore, glass bulb thermometers are not as easy to read and, hence, measurements are prone to human error, particularly when made under poor lighting conditions or when read by harried personnel.
Various attempts have been made to minimize or eliminate these deficiencies of the glass bulb thermometer by using temperature sensing probes that are designed to operate in conjunction with direct reading electrical thermometer instrumentation. In one such approach, an electronic temperature sensitive device, such as a thermistor, is mounted at the end of a probe and inserted into the patient. The change in voltage or current of the device, depending on the particular implementation, is monitored and when that output signal stabilizes, a temperature is displayed in digital format. This is commonly referred to as the direct reading approach and while it reduces the possibility of error by misreading the measured temperature, it may still require a relatively long period of time in which to reach a stabilized temperature reading. In the typical direct reading approach or mode, anywhere from three to five minutes are required for obtaining a temperature reading.
An inherent characteristic of electronic thermometers is that they do not instantaneously measure the temperature of the body to which they are applied. It may take a substantial period of time before the temperature indicated by the thermometer is representative of the actual temperature of the body measured. This lag is caused by the various components of the measurement system that impede heat flow from the surface of the body to the temperature sensor. Some of the components are the sensor tip, the skin and tissue of the body, and any hygienic covering applied to the sensor tip to prevent contamination between measurement subjects. This approach therefore provides only a partial solution.
One attempt to overcome the above-described deficiencies involves the use of a temperature sensitive electronic probe coupled with prediction or estimation circuitry to obtain a direct digital display of the patient's temperature before the probe has reached equilibrium with the patient. With this approach, assuming the patient's temperature is not significantly changing during the measurement time, the temperature that will prevail upon thermal stabilization of the electronic thermometer with the patient is predicted from measured temperatures and is displayed before thermal stabilization is attained. In many prior devices, prediction of temperature is performed by monitoring the measured temperature over a period of time, computing derivatives, and processing these variables to predict the patient's temperature.
With an electronic thermometer that operates by predicting the final, stable temperature, an advantage is that the temperature measurement is completed before thermal stabilization is attained, thereby reducing the time required for measurement. This would lessen the risk that the patient would not hold the probe in the correct position for the entire measurement time and requires less time of the attending medical personnel. Another advantage is that, because body temperature is dynamic and may significantly change during the two to eight minute interval associated with traditional mercury glass thermometer measurements, a rapid determination offers more timely diagnostic information. However, a disadvantage with such a thermometer is that the accuracy with which temperature is predicted declines markedly unless the processing and analysis of the data are accurately performed.
Electronic thermometers using predictive-type processing and temperature determination may include a thermistor as a temperature-responsive transducer. The thermistor approaches its final stable temperature asymptotically with the last increments of temperature change occurring very slowly, whereas the major portion of the temperature change occurs relatively rapidly. Such a temperature response is shown in FIG. 1. A graph of measured temperature 20 plotted as a function of measurement time 22 and temperature 24 for a typical thermistor is shown. As discussed above, the temperature 20 indicated by the thermistor lags the actual temperature TF 26 of the subject being measured. This lag can be seen by comparing the measured temperature line 20 to the subject's actual temperature line 26. It can be seen that as the measurement progresses from a start time, t0, the temperature rapidly increases from TR to T1 between times to t0 t1. The rate of increase in the indicated temperature is reduced between times t1 and t2, and the temperature line gradually tends toward the stabilization temperature TF 26 asymptotically as the time increases even more. As discussed above, the present invention is directed to a system capable of analyzing the temperature data gathered during an early period of the measurement, for example, between times t1 and t2, and predicting the final temperature TF. Prior attempts have been made to monitor that initial, more rapid temperature change, extract data from that change, and estimate the actual temperature of the tissue that is contacting the thermistor at that time, long before the thermistor actually stabilizes to the tissue temperature.
A prior approach used to more rapidly estimate the tissue temperature prior to the thermistor reaching equilibrium with the patient is the sampling of data points of the thermistor early in its response and from those data points, predicting a curve shape of the thermistor's response. From that curve shape, an asymptote of that curve and thus the stabilization temperature can be estimated. To illustrate these concepts through an example of a simpler system, consider the heat transfer physics associated with two bodies of unequal temperature as shown in FIG. 2, one having a large thermal mass and the other having a small thermal mass, placed in contact with each other at time=0. As time progresses, the temperature of the small thermal mass and the large thermal mass equilibrate to a temperature referred to as the stabilization temperature. The equation describing this process is as follows:
                                                                        T                ⁡                                  (                  t                  )                                            =                                                T                  R                                +                                                      (                                                                  T                        F                                            -                                              T                        R                                                              )                                    ·                                      (                                          1                      -                                              ⅇ                                                  -                                                      (                                                          1                              τ                                                        )                                                                                                                )                                                                                                                          =                                                T                  F                                -                                                      (                                                                  T                        F                                            -                                              T                        R                                                              )                                    ·                                      ⅇ                                          -                                              (                                                  1                          τ                                                )                                                                                                                                                    (                  Eq          .                                          ⁢          1                )            where: T(t) is the temperature of the smaller body as a function of time,                TF is the stabilization temperature of the system,        TR is the initial temperature of the smaller body,        t is time, and        τ is the time constant of the system.        
From this relationship, when the temperature T is known at two points in time t, for example T1 at time t1 and T2 at time t2, the stabilization temperature TF can be predicted through application of Equation 2 below.
                                                                        T                F                            =                                                                    T                    2                                    -                                                            T                      1                                        ⁢                                          ⅇ                                                                                                    t                            2                                                    -                                                      t                            1                                                                          τ                                                                                                              1                  -                                      ⅇ                                                                                            t                          2                                                -                                                  t                          1                                                                    τ                                                                                                                                              =                                                                                          T                      2                                        ⁢                                          ⅇ                                                                        t                          2                                                τ                                                                              -                                                            T                      1                                        ⁢                                          ⅇ                                                                        t                          1                                                τ                                                                                                                                  ⅇ                                                                  t                        2                                            τ                                                        -                                      ⅇ                                                                  t                        1                                            τ                                                                                                                              (                  Eq          .                                          ⁢          2                )            
Further, for a simple first order heat transfer system of the type described by Equation 1, it can be shown that the natural logarithm of the first time derivative of the temperature is a straight line with slope equal to −1/τ as follows:
                              ln          ⁢                                          ⁢                      (                                          ⅆ                T                                            ⅆ                t                                      )                          =                  K          -                      1            τ                                              (                  Eq          .                                          ⁢          3                )            and also:TF=T(t)+τ·T′(t)   (Eq. 4)where:
                    τ        =                  -                                                    T                ′                            ⁡                              (                t                )                                                                    T                ″                            ⁡                              (                t                )                                                                        (                  Eq          .                                          ⁢          5                )            where K=a constant dependent upon TR, TF, and τ,                T′=first derivative of temperature        T″=second derivative of temperature        
Prior techniques have applied these simple first order relationships to the analysis of the temperature equilibration curve. In some cases use has been made of thermistor time constants established by the thermistor manufacturer. However, all these techniques have failed to recognize that the temperature response curve cannot be accurately modeled as first order since it is determined by the complex thermodynamic interactions of the patient's tissue and vascular system with the hygienic probe cover, sensor, and probe stem. When the thermometer is placed in contact with body tissue, such as a person's mouth for example, the response curve is affected by the physical placement of the probe in relation to that tissue, by the heat transfer characteristics of the particular tissue, by the hygienic probe cover 34 (FIG. 2) that separates the probe from the tissue, and by heat transfer through the probe sensing tip and shaft 36, as is shown in FIG. 3. Each of these factors 36 in FIG. 3 affect the flow of heat from the thermistor and each possesses distinct thermodynamic qualities including thermal resistance and heat capacity. The biological factors 38 affect the flow of heat to the thermistor and vary significantly between patients, in particular with age and body composition. The factors, combined with the spatial geometry of the structures, cause the temperature sensed at the thermistor to follow a more complex characteristic curve than is predictable from a simple model such as that obtained using a priori factory-supplied time constant of the thermistor alone.
Previous estimation techniques have depended on the assumption that the temperature rise following skin contact followed an exponential curve (so called Newton “heating”). Such a model would be accurate under conditions where an infinite and well-stirred source of heat was available to warm the sensor, again as illustrated by FIG. 2. A probe cover 34 is mounted over the temperature sensor or probe 32 which is immersed in a large source of water 30 with specific heat and an initial temperature “Tw(0)”. The probe has a thermal mass “M” and an initial temperature “Tp(0)”. The probe cover has a thermal resistance “R”. Under these ideal conditions the flow of heat from the water bath to the probe is controlled by the simple equation:
                              Q          .                =                                                            T                W                            -                              T                P                                      R                    =                                    Δ              ⁢                                                          ⁢              T                        R                                              (                  Eq          .                                          ⁢          6                )            where {dot over (Q)}=flow_of_heat
Solving the differential equation for probe temperature at any time yields an equation of the form following for the probe temperature at any time “t” following immersion in the water:
                                          T            P                    ⁡                      (            t            )                          =                                            T              W                        ⁡                          (              0              )                                -                                    [                                                                    T                    W                                    ⁡                                      (                    0                    )                                                  -                                                      T                    P                                    ⁡                                      (                    0                    )                                                              ]                        ·                          ⅇ                              t                                  R                  ·                  M                                                                                        (                  Eq          .                                          ⁢          7                )            
The “time constant” of the thermal rise is determined largely by “M” the product of the thermal mass of the probe and “R” the thermal resistance of the probe cover.
Application of this simple model to the warming of a temperature probe placed in contact with a portion of the body such as the mouth or the axilla fails to account for the finite heat capacity of the tissues in the immediate region of the probe and for the thermal resistance of the successive layers of tissue beginning with epidermal layer and proceeding to the inner structures.
In particular, as the probe temperature increases, heat from the immediate region in contact with the probe has been removed requiring additional heat energy to travel through more tissue in order to reach the probe. This “remote” heat energy thus has a longer “time constant” than heat energy that has flowed into the probe from more proximate regions.
At any time “t”, there will be a temperature difference between the current value and the final value given by:
                                                        ⅆ                              ⅆ                t                                      ⁡                          [                              ⅇ                                                      -                    t                                                        R                    ·                    M                                                              ]                                            ⅇ                          t                              R                ·                M                                                    =                  -                      1                          R              ·              M                                                          (                  Eq          .                                          ⁢          8                )            Thus the limits of Equation 2 to model the complex conduction in body tissue may be seen by considering that the “rate of change” of temperature it predicts remains a constant proportion of the temperature change remaining to occur at any point in time.
The need therefore has arisen for a measurement system that can predict stabilization temperatures and can adapt to the changing heat flow characteristics of both the body under measurement and the measurement system itself, unlike a first order model. Prediction techniques have been proposed that use sets of simultaneous equations solved in real time to yield a likely temperature rise curve that indicates the stabilization temperature. To be successful, such techniques require use of equations with substantial numbers of coefficients so that the shape of the rise curve can be adequately approximated. Practical constraints limit the number of terms that can be employed and thus impose limits of the accuracy of such approaches. Furthermore, the computational solution of these equations is not a trivial matter when relatively simple, low power, microprocessor circuitry is used in the thermometer.
It is also noteworthy that while manufacturers can develop very sophisticated medical devices, the question of cost must be constantly kept in mind. Manufacturers strive to keep the cost of medical equipment as low as possible so that they can be made available to a wide variety of patients. While a thermometer with a much larger processor, with much faster computational speed, with much larger memory size could be made available so that computations could be performed faster and many more computations could be performed, the question of cost arises. Such increase in processing power would substantially increase the cost of a thermometer and may consequently make it unavailable to many patients. Instead, those skilled in the art desire a thermometer that is cost efficient but through the use of robust, accurate, and rapidly executing algorithms, is able to provide through sophisticated temperature data processing, accurate and fast prediction of the patient's temperature.
A need has also been recognized for a single thermometer that can measure temperature at the oral, rectal, and axilla sites of a patient. Various factors may come into play with a particular patient that make one or more of these sites unavailable for use in temperature measurement. Therefore a thermometer that can measure all three sites would provide a desired advantage over the need to find different thermometers for different sites. It should be recognized that measuring the temperature at the axilla site of a patient differs significantly from the oral and rectal sites. The temperature response of a probe to the axilla site is in most cases much different from the oral and rectal sites. Due to the fact that this site is composed of non-mucous epidermal tissues with an underlying stratum of fatty tissues, the curvature of the temperature response of a probe located in the axilla is much flatter than that of oral and rectal sites (see FIG. 6 where curve 100 is typical for an oral site and curve 102 is typical for a axillary site).
While prior predictive thermometry techniques have advanced the art of electronic thermometry significantly, those skilled in the art have recognized that a need still exists for an electronic thermometer that can predict a stabilization temperature at an early stage of the measurement process where measurement conditions and the characteristics of the subject under measurement vary from measurement to measurement. Additionally, it has been recognized that a need exists for a single thermometer that can measure and predict the patient's temperature from multiple sites, such as all of the oral, rectal, and axilla sites. Further, a need exists for a medical thermometer that is accurate, yet comprises relatively simple, inexpensive circuitry. The invention fulfills these needs and others.